Special Astrophysics Seminar

Mass and Radius of Neutron Stars from Modeling Pulse Shapes of Millisecond Pulsars

Denis Leahy

University of Calgary

Millisecond pulsars are rapidly rotating neutron stars where general relativity plays a strong role in the propagation of light from the neutron star to observer and formation of observed pules shapes. Thus pulse shapes carry information on the mass, radius and surface shape of the neutron star. Comparison of calculations of pulse shapes with observed pulse shapes can give useful constraints on mass and radius, which constrain the equation of state (EOS) of matter at high density. Here, calculations are described for the modeling of X-ray pulse shapes. These include General Relativity for the neutron star metric and for geodesics for photons in that metric; time-delays and Doppler effects for photons; and oblateness of the neutron star surface, which results from the rapid rotation. Frame dragging is also calculated bit shown to be not important for observed rotation periods. The modeling has been applied to Rossi X-Ray Timing Explorer observations of the millisecond pulsars XTE J1814-334 and SAX J1808-3658. The most recent results include modeling multi-epoch pulse shapes from the 1998, 2002 and 2005 outbursts of SAX J1808-3658. It is a challenging test for the model to fit these greatly different pulse shapes. A consistent neutron star model can work only if the model includes an extra component: light scattered from the accretion disk. The new mass and radius constraints are for SAX J1808-3658 are presented and compared to those for other neutron stars.